New habits are measured in months, not “21 days”

12. New habits are measured in months, not “21 days”

Cost: No cost at all. What’s required is simply repeating the same action every day in the same context for several months.

In plain terms: The idea that “it takes 21 days to form a habit” has no scientific basis. One study tracking participants over 12 weeks found that it took as little as 18 days for an action to become automatic, while some people needed as many as 254 days. It’s perfectly normal for things to feel difficult for two or three weeks. Missing a single day also does not reset any progress.

Benefit: Ninety-six volunteers each chose a simple action related to eating, drinking, or physical activity, and performed it daily in a fixed context (such as “after breakfast”) for 12 weeks, rating how automatic the action felt each day. Data from 82 participants proved sufficient for analysis, and the model successfully matched 62 of them. The number of days required to reach 95% automation ranged from 18 to 254 days. The study also showed that missing one day of practice has no meaningful impact on habit formation.

Evidence grade: B

Sources: Lally P, van Jaarsveld CHM, Potts HWW, Wardle J (2010). How are habits formed: Modelling habit formation in the real world. European Journal of Social Psychology, 40(6), 998–1009. <https://doi.org/10.1002/ejsp.674>

Notes: This is the only study of its kind, involving just 96 participants who performed very simple actions (like eating a piece of fruit after a meal). No research has examined how long it takes to form more complex habits, such as running three times a week or writing daily. The main takeaway here is to set realistic expectations: finding it hard for two weeks is not a sign of failure. To actually make repetition happen, link the action to a fixed context — see point 1 in this section, which advises specifying “at what time, where, and under what circumstances” the action should be done.